2,6-Dimethyl-5-hepten-1-al is of great commercial interest as fragrance or as flavor due to its characteristic organoleptic properties. In particular, 2,6-dimethyl-5-hepten-1-al is used as additive in cosmetic preparations as well as in laundry and fabric detergents. Furthermore 2,6-dimethyl-5-hepten-1-al is a highly valuable intermediate for the production of other fragrances and flavors such as 6-hydroxy-2,6-dimethylheptanal and 6-methoxy-2,6-dimethylheptanal.
2,6-Dimethyl-5-hepten-1-al can be isolated from natural sources for instance from Java Citronella oil. However, the isolation of fragrances from natural sources is mostly expensive, their available amount is often limited and, on account of fluctuations in environmental conditions, they are also subject to variations in their content, purity etc.
Thus, a number of synthetic methods for the production of 2,6-dimethyl-5-hepten-1-al have been developed.
U.S. Pat. No. 4,242,281 for instance describes an industrial process for the production of racemic 2,6-dimethyl-5-hepten-1-al with a purity of 85% by a Darzens reaction, where 6-methyl-5-hepten-2-one is reacted with ethylchloroacetate in the presence of an alkali metal alkoxide such as sodium methoxide.
Corma et al., Journal of Catalysis, 2005, Vol. 234, pp. 96-100, describe a halogen-free synthesis strategy for the preparation of racemic 2,6-dimethyl-5-hepten-1-al involving the chemoselective oxidation of citral with H2O2 using a Sn-Beta zeolite based catalyst.
Burger et al., Journal of Chemical Ecology, 2002, Vol. 28, No. 12, pp. 2527-2539, describe the synthesis of (R)- and (S)-2,6-dimethyl-5-hepten-1-al starting from (R)- and (S)-3,7-dimethyl-1,6-octadiene, respectively. The synthesis comprises the selective epoxidation of the internal triple-substituted double bond of (R)- or (S)-3,7-dimethyl-1,6-octadiene using 3-chloroperbenzoic acid followed by the oxidation of the terminal double bond with ozone and the reduction of the thus obtained oxidation products with zinc to yield (R)- or (S)-2,6-dimethyl-5-hepten-1-al, respectively.
Since these processes have several technical and/or economical disadvantages, there is a need to find alternative synthetic processes, which allow the production of 2,6-dimethyl-5-hepten-1-al on industrial-scale in a more efficient way.
WO 2010/076182, describes a process for producing ketones, including the reaction of 1,1-disubstitued olefins with N2O in the presence of a solvent comprising at least one proton-supplying functional group.
Romanenko et ala, Russian Chemical Bulletin, International Edition, 2007, Vol. 56 (6), pp. 1239-1243, describe the chemoselective oxidation of limonene with N2O yielding 4-acetyl-1-methylcyclohexane as the major product.
Semikolenov et al., Russian Chemical Bulletin, International Edition, 2005, Vol. 54 (4), pp. 948-956, describe the oxidation of terminal olefins like for instance 1-butene with N2O. In the case of 1-butene the aldehyde with one carbon atom less (propionaldehyde) is formed as a minor product with a selectivity of only 29%. Longer chain terminal olefins like 1-hexene and 1-octene behave similarly yielding the corresponding aldehydes with a selectivity of only 27% and 26%, respectively. In all cases the terminal olefins do not contain further oxidizable double bonds.